Late Blight in Potatoes

Late blight can be an extremely destructive potato disease in the Prairies. Unlike the name may suggest, the disease can attack both tubers and foliage at any stage of crop development. Under favourable conditions if no controls are implemented, entire fields can be destroyed within days.

Late Blight in Potatoes Late Blight in Potatoes

Host Crops

Potato and Tomato


Under warm conditions with high humidity or wet foliage, which are favourable for disease development, the fungus can spread rapidly through the crop, causing complete and rapid blighting of foliage. The tubers can be infected while they are in the ground, at harvest, or in storage.

The spores are carried to nearby fields primarily by wind. They require water to germinate and penetrate the potato tissue. This can be provided by rain, dew, sprinkler irrigation and very high relative humidity. The lesions on leaves and stems become visible as small flecks within a few days after infection, expanding to form water-soaked, grey-green areas on the leaf. These lesions may be surrounded by a circle of light green tissue. The dying tissue turns grey to tan and becomes dry within a few days. The disease may also develop on leaf petioles and stem tissue.

Under moist conditions, spore production begins. The fungus may appear as a white, mildew-like growth at the edge of the lesion, usually on the underside of the leaf. This white growth distinguishes late blight from several other foliar diseases of potatoes.

The spores are carried by wind and rain to healthy plants where the disease cycle begins again. The fungus can complete many reproductive cycles in a season. Thus, once established, the disease may spread rapidly.

Tubers are infected by spores washed from lesions to the soil and also by contacting spores at harvest. Tuber infections are characterized by patches of irregular, small to large, slightly depressed areas of brown to purplish skin. Cutting just below the skin reveals a dark, reddish-brown, dry, corky rot.

Symptoms Of Damage

Field scouting should be concentrated in areas of the field most likely to have high moisture, dew, or relative humidity for the greatest length of time (e.g., low spots, areas shaded by windbreaks) or in areas that are difficult to access with sprayer equipment.

Computer-based programs are available to track weather conditions and help predict or forecast when the disease may occur. Disease forecasting programs predict when environmental conditions favour disease and recommend when fungicide applications may be needed. As the risk of disease becomes greater, crop monitoring should be intensified and also fungicide application frequency.

Early symptoms are small, light to dark green, circular to irregular-shaped, water-soaked spots. The late blight disease can infect any part of the plant and new infections coming in often appear on the upper leaves of the plants, but can also start at any other part of the foliage.  From there the infections can spread to any part of the plant. If the disease is seed borne the symptoms may first appear on the lower canopy.

Lesions often begin to develop near the leaf tips or edges, where dew is retained the longest.

During cool, moist weather, these lesions expand rapidly into large, dark brown or black lesions, often appearing greasy. The lesions are not limited by leaf veins, and as new infections occur and existing infections coalesce, entire leaves can become blighted and die within a few days. The lesions may expand down petioles and stems of the plant.

Scouting Techniques

Late blight, requires a cool, moist environment. If infected leaves are examined in the early morning or during other cool damp weather, a white mildew growth may be seen on the underside of those leaves. A pale green to yellow border is often present around the lesions.

Plants severely affected by late blight have a distinctive odour resulting from the rapid breakdown of potato leaf tissue. A similar odour may occasionally be detected after chemical vine-kill or a severe frost.

Late blight infection of tubers is characterized by brown to purplish irregularly shaped, slightly depressed areas of variable size on the skin. These symptoms may be less obvious on russet and red-skinned cultivars. A tan to reddish-brown, dry, granular rot is found under the skin in the discoloured areas, extending into the tuber, usually less than 12 mm (1/2 in.). The extent of rot in a tuber depends on the susceptibility of the cultivar, temperature, and the length of time after the initial infection. The margin of the diseased tissue is not distinct. The margin is marked by brown finger-like extensions into the healthy flesh of the tuber. Severely infected tubers are susceptible to extensive storage rots. The mildew-like growth of the causal fungus may appear on the surface of tubers.

Economic Thresholds

None available.

Control Tips

  • Effective control of this disease requires implementing an integrated disease management approach. The most important measures are cultural practices like planting only healthy, disease-free seed tubers, keeping a clean operation (destroy all cull and volunteer potatoes), avoiding frequent or night-time irrigation of potatoes, hilling to reduce the incidence of tuber infection, starting to harvest after the vines are completely dead (2 weeks after vine killing), managing cull piles, rotating crops, and ensuring that tubers are dry when placed in storage.
  • Fungicides are usually applied when potato plants are 15 - 30 cm (6-12 in.) high or when conditions have been determined to be favourable for infection. Fungicides are most effective if applied when the canopy begins to close within the row. Applications should continue, as needed, throughout the growing season. It is also important to rotate fungicide chemistries (modes of action) to manage emergence of fungicide resistant isolates.
  • Disease forecasting models that will lead to more effective timing of fungicides are available.
  • Complete coverage is essential when using protectant fungicides. If late blight is present in a field, a combination of protectant and systemic fungicides should be used. Consult the provincial Guide To Crop Protection for information on currently approved fungicides.

For further information, contact an ARD/MASC Service Centre